1.5 kind of makes sense. It holds Kingdom Hearts 1 and a remake of the GBA game that comes before Kingdom Hearts 2 (also comes with a the cutscenes of another game between 1 and 2). I guess they wanted to keep the naming similar because 2.5 comes with Kingdom Hearts 2 and...a prequel to every game... and a cutscene collection of some weird side game about an AI. They then released a bundle of both of these on PS4 which they named 1.5+2.5. 2.8 they were really scrambling. Because they wanted to release the rest of the games in some way before Kingdom Hearts 3 so that holds a sequel to Kingdom Hearts 2, A standalone demo for 3 and...a movie from the earliest part of the series that actually is a giant pit of questions with no answers which may or may not have been answered by the gacha. No idea.